Disk array system and rebuild method thereof

ABSTRACT

Disclosed is a disk array system which includes a plurality of disks constituting a disk array and a backup storage for backing up data in the disk array and performs control so that when a failed disk among the disks constituting the disk array is replaced with the replacement disk, restoration of data in the replacement disk is performed using the backup device.

FIELD OF THE INVENTION

The present invention relates to a redundant disk array system. Morespecifically, the invention relates to a disk array system and a rebuildmethod thereof.

BACKGROUND OF THE INVENTION

Disk arrays are also referred to as RAIDs (Redundancy Arrays ofIndependent (Inexpensive) Disks) and have a configuration in which aplurality of disks are arranged in an array form. By appendingredundancy data to data, reliability of the disk array is improved. TheRAID includes a RAID0 in which by striping, for example, a plurality ofdisks are used as if it were one disk, a RAID1 in which using mirroring,the same data is written to a plurality of disks, a RAID5 in which byrecording data and parities on a plurality of disks, fault tolerance isimproved, and the like. Among these disks, in the RAID5, parityinformation for error correction is generated when the data is writtento the disks. This parity information is distributed across and writtento the disks with the remaining data by the striping. The number of thedisks required for the parities is set to one. With this arrangement,even if one disk has failed, the data can be restored using the data andthe parities in the remainder of the disks.

When a RAID controller detects a failure of a disk in conventional diskarrays, the conventional disk arrays have a function of performingreplacement to a spare disk and rebuilding (rebuilding) data on thefailed disk onto the replacement disk. On that occasion, using disksother than the replaced disk among plural disks constituting the diskarray, data on the replacement disk is prepared.

In a disk array system for which high availability is required, it isnecessary to rebuild data onto a replacement disk after replacement of afailed disk, in parallel with a normal operation (unless the normaloperation is performed at the time of rebuilding, business isdisturbed). At the time of parity rebuilding after the replacement ofthe disk, it is necessary to read data from all of disks that belongs toa parity group, other than the replaced disk. Thus, access performanceis normally degraded. When the number of data drives is increased in aconfiguration such as the RAIDs constituting the parity group, theperformance will be further degraded.

FIG. 10 is a schematic diagram showing a typical configuration exampleof a conventional disk array system. Referring to FIG. 10, an accessmodule 14 serves as an interface for transmitting and receiving acommand, data, and a control signal between a host 20 and a disk array.A RAID module 13 is constituted from the RAID controller and managementsoftware, and performs a read/write, fault detection, processing fordisk rebuilding, and the like on data on a plurality of disks 11 ₁ to 11_(n) of a redundant configuration. A disk access bus 12 providesconnecting paths between a plurality of disk drives 11 ₁ to 11 _(n) andthe RAID controller 13. Incidentally, rebuilding of a disk drive isperformed on the array of the redundant configuration such as the RAID1, 5, or 10. Each of the disk drives 11 ₁ to 11 _(n) is constituted fromthe disk drive such as an HDD (Hard Disk Drive), and is abbreviated as a“disk”.

Patent Document 1 discloses a configuration in which in order tosuppress further degradation of the performance when a number of datadrives n is increased in an nD+1P representing a disk configuration suchas the RAID constituting the parity group, copying is performed from onedisk regardless of the number n. Patent Document 2 describes rebuilding(rebuilding) onto a disk by the RAID controller. A rebuild rate (RebuildRate) of a failed disk of 100% means that the system is all applied tothe rebuilding of the failed disk. The rebuild rate of 0% means that thesystem performs rebuilding of the failed disk during an idle time whenthe system performs no other operation. A default rebuild rate is set toapproximately 30% or the like, for example.

[Patent Document 1]

JP Patent Kokai Publication No. JP-P2002-108571A

[Patent Document 2]

JP Patent Kokai Publication No. JP-P2004-38290A

SUMMARY OF THE DISCLOSURE

In a redundant disk array system having a RAID level 5 or the like, datain a failed disk (replacement disk) is prepared using disks other thanthe failed disk at the time of disk rebuilding, it takes time to performthe rebuilding. With an increased capacity of disks in particular, thetime for the rebuilding is increased. Further, during the rebuilding, aredundancy level is reduced. The access performance of the disk arraysystem during the period of the rebuilding is thus degraded.

Accordingly, it is an object of the present invention to provide a diskarray system and a method that achieve a higher speed operation forrebuilding of a replacement disk and shorten a period during which theredundancy level is reduced.

Other object of the present invention is to provide a disk array systemand a method that suppress degradation of access performance duringpreparation of data on a replacement disk.

The above and other objects are attained by the present invention whichis generally configured as follows.

A system according to one aspect of the present invention includes abackup device for backing up data in the disk array system, andrestoration of data in a replacement disk drive is performed using thebackup device. Other disk drives constituting the disk array system arenot thereby used for the restoration.

A disk array system according to the present invention comprises: aplurality of disk drives constituting a disk array; a backup storagedevice; a backup module for performing control so that data in said diskarray is backed up onto said backup storage device; and a rebuild modulefor performing rebuilding of data in a replacement disk drive, withwhich a failed disk drive among said disk drives constituting said diskarray is replaced, based on the data backed up in said backup storagedevice.

A storage control device according to another aspect of the presentinvention comprises: a plurality of storage units; a backup deviceprovided for said storage units; backup means for performing control sothat data in said storage units is backed up onto said backup device;and control means for performing control so that when a failed storageunit among said storage units is replaced with a replacement storageunit, rebuilding of data in a replacement storage unit is performedbased on the data backed up in said backup device.

A method according to another aspect of the present invention,comprises:

backing up data in a disk array including a plurality of disk drivesonto a backup storage device which is provided separately from said diskarray, and performing control so that restoration of data in areplacement disk drive at a time of replacement of a failed disk driveamong the disk drives constituting the disk array is performed by thebackup storage device.

In the present invention, it may be so arranged that when the data isupdated by an access from a host, the information of update location isrecorded on a modify map (also referred to as a “differential map”).Only the update portion updated after the backup onto the backup storagedevice may be sent to the backup storage device, and the backup storagedevice may back up the changed portion.

In the present invention, the data stored in the failed disk drive maybe read from the backup storage device and written to the replacementdisk drive. Then, after writing of the backup data in the failed diskdrive to the replacement disk drive from the backup storage device isfinished, the modify map may be referred to and data in a data block onthe failed disk drive updated after the backup may be generated, basedon the data on other disk drives, and the generated data may be writtento the replacement disk drive.

In the present invention, the modify map may be referred to, and writingback from the backup storage device may not be performed on an updateddata block of the failed disk drive.

In the present invention, an update performed on the disk drivecorresponding to the failed disk drive during rebuilding may be directlywritten to the replacement disk drive without being backed up by thebackup storage device.

In the present invention, even before the rebuilding is finished, withrespect to a data block that has become the latest data among datablocks of the replacement disk drive under the rebuilding, access to thereplacement disk drive may be authorized.

In the present invention, a disk access bus/switch for data transferbetween said backup storage device and said replacement disk drive isprovided and the rebuilding of said replacement disk drive from saidbackup storage is performed through the rebuild module provided inparallel with a RAID module for performing a normal read/write access tosaid disk array and said disk access bus/switch, thereby said rebuildingoperation of said replacement disk drive being prohibited frominfluencing a normal access by said RAID module said to said disk array.

The meritorious effects of the present invention are summarized asfollows.

According to the present invention, except for data updated afterbackup, other disk drive in a disk array is not accessed duringrebuilding. Thus, degradation in performance will not be caused duringthe rebuilding.

According to the present invention, the degradation in the performancedoes not occur during the rebuilding. Accordingly, the rebuilding doesnot need to be performed at a low speed. For this reason, a rebuildingperiod can be reduced. Then, according to the present invention, due toa reduction in the rebuilding period, a probability of occurrence of afailure in a second disk during the rebuilding can be reduced.

Still other features and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description in conjunction with the accompanying drawingswherein only the preferred embodiments of the invention are shown anddescribed, simply by way of illustration of the best mode contemplatedof carrying out this invention. As will be realized, the invention iscapable of other and different embodiments, and its several details arecapable of modifications in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawing and descriptionare to be regarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an embodiment of thepresent invention;

FIG. 2 is a flow diagram for explaining WRITE processing at a normaltime in the embodiment of the present invention;

FIG. 3 is a flow diagram for explaining READ processing in theembodiment of the present invention;

FIG. 4 is a flow diagram for explaining differential backup processingin the embodiment of the present invention;

FIG. 5 is a flow diagram for explaining processing before and after acrash in the embodiment of the present invention;

FIG. 6 is a flow diagram for explaining WRITE processing from the crashto completion of rebuilding in the embodiment of the present invention;

FIG. 7 is a flow diagram for explaining READ processing from the crashto completion of rebuilding in the embodiment of the present invention;

FIG. 8 is a flow diagram for explaining processing for rebuilding in theembodiment of the present invention;

FIG. 9 is a diagram showing a configuration example of update bits; and

FIG. 10 is a diagram showing a typical configuration of a conventionaldisk array.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will be described withreference to appended drawings. In the present invention, a backupdevice is prepared for a plurality of storage units. Data in the storageunits is backed up by the backup device. Rebuilding of data in areplacement storage unit at a time of replacement of a failed storageunit of the storage units is performed, based on the data backed up bythe backup device. Preferably, the storage units in an embodiment of thepresent invention constitute a disk array.

FIG. 1 is a diagram showing a system configuration according theembodiment of the present invention. Referring to FIG. 1, a disk array10 includes a plurality of disk drives 11 ₁ to 11 _(n), disk accessbus/switch 18, a RAID module 13, an access module (host adapter) 14, abackup module (Backup Module) 15, a modify map (Modify Map) 16, and arebuild module (Rebuild Module) 17. The RAID module 13 and the rebuildmodule 17 make access for writing to and reading from a plurality ofdisks through the disk access bus/switch 18. A backup storage 30 isconnected to the disk array 10 through a predetermined interface. Thebackup storage 30 may be of course configured to be the disk array. Eachof the disk drives 11 ₁ to 11 _(n) is constituted from the disk drivesuch as an HDD (Hard Disk Drive), and is simply abbreviated as a “disk”.

In the present invention, the backup module 15 periodically backs updata in the disks 11 ₁ to 11 _(n) constituting the disk array onto thebackup storage 30. When the access module 14 receives a backup commandfrom a host 20, the backup module 15 reads data from one of the diskdrives 11 ₁ to 11 _(n) through the RAID module 13 (and error detectionand correction processing including a parity check and the like isperformed) and writes read data in the backup storage 30.

When modification of a data block is performed within a period from astart of backup processing by the backup module 15 or from completion ofthe backup processing to next backup, the access module 14 sets in amodify map 16 information (of one bit) indicating that the modificationof the data block has been performed. The modify map 16 may beconfigured to include an update bit (update bit) of one bit and arebuild bit (rebuild bit) per logical block (of 1 K byte, for example),for each disk (of which a description will be described later withreference to FIG. 9). Alternatively, the modify map 16 may be configuredto include the update bit of one bit per extent. The modify map 16 isstored in a memory module such as a DRAM (dynamic random access memory)or an EEPROM (electrically erasable programmable ROM) that is anonvolatile memory for holding recorded data even at a time of poweroff.

Assume that one of the disks constituting the disk array has failed, thefailed disk is replaced with a spare disk or the like, and that data isrebuilt onto the replacement disk in this embodiment mode. Then, it ispreferable that transfer of the data from the backup storage 30 to thereplacement disk is performed through the rebuild module 17 and the diskaccess bus/switch 18 so as not to influence a normal access from thehost 20 to other disk. That is, the normal access from the host 20 toother disk is performed through the access module 14, RAID module 13,and disk access bus/switch 18. Processing for the rebuilding from thebackup storage 30 is performed through the rebuild module 17 provided inparallel with the RAID module 13 and the disk access bus/switch 18. APort for a cross bus switch or the like constituting the disk accessbus/switch 18 may be different between the RAID module 13 and therebuild module 17. Alternatively, different access buses constitutingthe disk access bus/switch 18 may be provided for the RAID module 13 andthe rebuild module 17, respectively. With this arrangement, influence ofthe processing for the rebuilding on the normal access is suppressed asmuch as possible. Details of an operation of a disk array system shownin FIG. 1 will be described in connection with embodiments below.

Embodiment

FIG. 2 is a flow diagram for explaining write processing at a normaltime in this embodiment. The access module 14 receives a WRITE (write)request from the host 20 (at step S11). Incidentally, though noparticular limitation is imposed, the HDD (Hard Disk Drive) is employedfor each of the disks 11 ₁ to 11 _(n) of FIG. 1 in FIGS. 2 through 8.

The access module 14 commands the RAID module 13 to perform a WRITE(transmits the WRITE command) (at step S12).

The RAID module 13 performs redundancy processing (parity generation) ofdata at a parity generator not shown (at step S13).

The RAID module 13 writes data and a parity which have been maderedundant into a disk (HDD) (at step S14).

The access module 14 sets the update bit (update bit) of the modify map16 corresponding to the logical block with data written therein (at stepS15).

FIG. 3 is a flow diagram for explaining READ processing at a normal timein this embodiment. The access module 14 receives a READ (read) requestfrom the host 20 (at step S21).

The access module 14 commands the RAID module 13 to perform a READ(transmits a READ command)(at step S22).

The RAID module 13 reads data in a logical block specified from a disk(HDD) and transfers the data to the access module 14 (at step S23).

The access module 14 transmits the data read from the disk array to thehost 20 (at step S24).

FIG. 4 is a flow diagram for explaining differential backup processingin this embodiment. Differential backup is the processing forselectively backing up only a portion (such as a logical block) whichhas been updated after backup processing. The differential backupprocessing will be described below.

The backup module 15 searches for the logical block of the modify map 16with the update bit set therein (at step S31).

When the logical block of the modify map 16 with the update bit (Updatebit) set therein is present, the following processing is repeated (atstep S32).

The update bit (Update bit) in the logical block of the modify map 16searched for is cleared (at step S33). Clearing of the update bitindicates that updating has been performed.

The backup module 15 reads data in the searched logical block throughthe RAID module 13 (at step S34).

The backup module 15 transfers the read data in the block to the backupstorage 30 (at step S35).

The backup storage 30 records the transferred data (at step S36).

FIG. 5 is a flow diagram for explaining processing before and after adisk (HDD) has been crashed.

Due to occurrence of the crash in the disk (HDD) at a normal operation,an operation at a time of a fault is started (at step S42).

The crashed disk (HDD) is replaced with a new disk (HDD) (at step S43).Incidentally, the new disk may be a spare disk prepared for in the diskarray. Further, the RAID module 13 may automatically perform replacementof a disk from which the fault has been detected or replacement of afailed disk to the spare disk.

Rebuilding of the replacement disk is started by the rebuild module 17(at step S44).

Data is written to the replacement disk (new HDD) from the backupstorage 30 (at step S45).

Using the modify map (Modify Map) 16, data in the logical block with theupdate bit set therein is generated, and the generated data is writtento the replacement disk (new HDD) (at step S46). For generation of thedata in the logical block, the data (or the parity thereof) is generatedfrom data in disks other than the replacement disk among disksconstituting the disk array and written to the replacement disk, byreferring to information (striping information on parity blocks and thedata) in the RAID module 13.

The rebuilding of the replacement disk is completed, and restoration tothe normal operation is performed (at step S47).

FIG. 6 is a flow diagram for explaining WRITE processing from occurrenceof a crash of a disk to completion of rebuilding. In this case, datawriting to disks other than the failed disk is performed, and write datais not backed up. Thus, the update bit (Update bit) of the modify map isset.

The access module 14 receives a WRITE request from the host 20 (at stepS51).

The access module 14 commands the RAID module 13 to perform a WRITE (atstep S52).

The RAID module 13, which has received the WRITE command, performsredundancy processing (at step S53).

The RAID module 13 writes data/parity that has been made redundant touncrashed disks (HDD) (at step S54).

The access module 14 sets in the modify map 16 the update bit (Updatebit) corresponding to the logical block with the data written therein,and resets (clears) the rebuild bit (Rebuild bit) for the logical block(at step S55).

FIG. 9 is a diagram showing an arrangement example of update bits(Update bits) and rebuild bits (Rebuild bits) in the modify map 16. Eachof the update bits (U) and each of the rebuild bits (R) are assigned toa logical block (of 1K byte), and (m+1)×n update and rebuild bits areassigned to n disk drives. A cache structure may be of course adoptedfor the modify map 16. Contexts of the modify map 16 are reset when asystem operation is started or the like. The contexts of the modify map16 may be saved on a disk or the like at power-off of the system.Alternatively, the modify map may be constituted from the EEPROM or thelike.

FIG. 7 is a flow diagram for explaining READ processing from occurrenceof a crash in a disk to completion of rebuilding of a replacement disk(a new HDD) in this embodiment.

The access module 14 receives a READ request from the host 20 (at stepS61).

The access module 14 commands the RAID module 13 to perform a READ (atstep S62).

It is determined whether a block targeted for reading is in a crasheddisk (HDD) or not (at step S63).

When it is determined that the block targeted for reading is inside thecrashed disk (HDD), the RAID module 13 reads data associated with thespecified block from uncrashed disks (at step S64).

The RAID module 13 computes and prepares data in the specifiedblock/parity block from the data (parity) associated with the specifiedblock (at step S65).

The RAID module 13 sends to the host 20 prepared data through the accessmodule 14 (at step S66).

On the other hand, when it is determined that the block targeted forreading is not in the crashed disk (HDD) at step S63, the RAID module 13reads the data in the specified block (at step S67) and sends to thehost 20 the read data through the access module 14 (at step S68).

FIG. 8 is a flow diagram for explaining processing for rebuilding by therebuild module 17 in this embodiment. The processing by the rebuildmodule 17 will be described below with reference to FIG. 8.

The rebuild module 17 sets a target block (logical block) in a disktargeted for rebuilding at a leading edge of the disk (at step S71).

The rebuild module 17 repeats the following processing until processingon all blocks (blocks) is completed (at step S72).

It is determined whether the target block is the parity block or not atstep S73. The rebuild module 17 determines whether the target block inthe replacement disk (new HDD) targeted for rebuilding is the parityblock or not, by referring to management information (not shown) held inthe RAID module 13, for example.

When it is determined that the target block is not the parity block inthe determination at step S73, the rebuild module 17 reads datacorresponding to the target block from the backup storage 30 (at stepS74).

The rebuild module writes the data read from the backup storage 30 tothe corresponding block (target block) in the replacement disk (newHDD)(at step S75).

Next, the rebuild module 17 changes the target block to the next block(at step S79).

When it is determined that the target block is the parity block at thedetermination at step S73, the rebuild module 17 reads data in a logicalblock associated with the target block from the backup storage 30 (atstep S76). In this embodiment, the parity block in the disk array is notstored in the backup storage 30 as the backup data at a time of backup.For this reason, when it is determined as a result of the determinationat step S73 that the target block to be written to the replacement disk(new HDD) is the parity block, the rebuild module 17 reads block data inother disks (disks other than the replacement disk, which constitutesthe array) that constitutes the disk array, associated with the targetblock, from the backup storage 30.

Then, the rebuild module 17 generates the parity data to be stored inthe target block from the data read from the backup storage 30 (blockdata in other disk associated with the target block) (at step S77).

Next, the rebuild module 17 writes the generated parity data to thecorresponding block (target block) in the replacement disk (new HDD) (atstep S78).

When it is determined that processing on all blocks is completed in thedetermination at step S72, the rebuild module 17 searches for a block(block that has not been rebuilt yet) associated with the block storedin the replacement disk (new HDD) with the update bit (update bit) settherein and with no rebuild bit (rebuild bit) set therein (at step S80).That is, the rebuild module 17 searches for the block that has beenmodified and has not been rebuilt yet after the start of the backup.

When the pertinent block exists after a result of the search (Yes atstep S81), the rebuild module 17 sets the rebuild bit (rebuild bit) ofthe modify map 16 corresponding to the searched block (at step S82).Setting of the rebuild bit (rebuild bit) of the logical block in themodify map 16 indicates that rebuilding of the logical block onto thereplacement disk has been completed.

The rebuild module 17 reads from other disks (HDDs) of the disk arrayblock data associated with the block that has been searched for (at stepS83).

From the data read from the other disks (HDDs), the rebuild module 17computes and prepares data to be stored in the replacement disk (newHDD) (at step S84).

The rebuild module 17 writes the prepared data to the replacement disk(new HDD) (at step S85).

When it is found that no block with the update bit (update bit) set andwith no rebuild bit (rebuild bit) set therein is present at step S81,the rebuild module 17 changes the state of the disk array to a normalstate (at step S85).

An operation of this embodiment was described below. Some of variationexamples of the embodiment will be described below.

It may also be so arranged that the rebuild module 17 refers to themodify map 16 and does not write back an updated block (block with theupdate bit (update bit) set therein) among blocks of a failed disk (newHDD), to the replacement disk (new HDD) from the backup storage 30. Itis because, as the updated block, data generated from other disks (HDDs)of the disk array is written, as shown in steps S83 through S85 in FIG.8, for example.

In another variation example, an update to the failed disk duringrebuilding of the replacement disk (new HDD) may also be written to thereplacement disk (new HDD) without alteration. When the access module 14receives a WRITE request from the host 20 during execution of therebuilding by the rebuild module 17, the WRITE command is issued to theRAID module 13, and writing to the replacement disk (new HDD) isperformed through the RAID module 13. The latest data will be written tothe block of the replacement disk (new HDD) after backup processing.Thus, the access module 14 does not set the update bit (update bit) ofthe modify map. On the other hand, the rebuild bit of the block may beset. For this reason, rebuilding of the data on the block from thebackup storage 30 by the rebuild module 17 is omitted (as in anoperation branched to NO in the determination at step S81 in FIG. 8).

In another variation example, with respect to access to the latest dataamong block data of the replacement disk (new HDD) being rebuilt, accessto the replacement disk (new HDD) may be authorized even beforecompletion of the rebuilding. With respect to the block with the rebuildbit of the modify map 16 set therein (with the update flag resettherein, however), the access module 14 performs control so that accessto the replacement disk (new HDD) is permitted in response to an accessrequest from the host 20.

Incidentally, the backup storage 30 may be of course a recording medium(such as a tape) other than the disk (HDD) in the embodiment describedabove. The backup storage 30 may be installed in a remote locationconnected through the network.

A fibre channel fabric switch or a cross bus switch may be employed asthe disk access bus/switch 18 in FIG. 1. In the case of a configurationdescribed above, access to a replacement disk (new HDD) and access toother disk can be non-blocked. Alternatively, a dual Fibre channelarbitrated loop may be used as the disk access bus/switch 18 in FIG. 1.One loop may be used for access to the replacement disk (new HDD), andthe other may be used for access to other disk (HDD). In the case ofsuch a configuration as well, access to the replacement disk and accessto other disk can be non-blocked.

A description was made in connection with the embodiment describedabove. The present invention is not limited to the configuration of theembodiment described above, and may of course include various variationsand modifications that could be made by those skilled in art within thescope of the present invention.

It should be noted that other objects, features and aspects of thepresent invention will become apparent in the entire disclosure and thatmodifications may be done without departing the gist and scope of thepresent invention as disclosed herein and claimed as appended herewith.

Also it should be noted that any combination of the disclosed and/orclaimed elements, matters and/or items may fall under the modificationsaforementioned.

1. A disk array system comprising: a plurality of disk drivesconstituting a disk array; a RAID module for performing a normalread/write access to the disk array; a backup storage device; a backupmodule for reading data from said disk array and writing the read dataonto said backup storage device; a replacement disk drive for replacingone of the plurality of disk drives after one of the plurality of diskdrives fails; a rebuild module for performing rebuilding of data on thereplacement disk drive based on the data backed up in said backupstorage device; and means for rebuilding data on the replacement diskdrive from the backup storage device through the rebuild module whilesimultaneously providing normal read/write access to the disk drivesthat have not failed through the RAID module.
 2. The disk array systemaccording to claim 1, further comprising a modify map for recordinginformation corresponding to an update location, when the data in saiddisk array is updated.
 3. The disk array system according to claim 2,wherein, when the data in said disk array is updated by an access from ahost after the backup onto said backup storage device, said backupmodule sends to said backup storage device the data updated after thebackup onto said backup storage device, said modify map records theinformation corresponding to the updated data; and said data updatedafter backup is stored as the backup data in said backup storage device.4. The disk array system according to claim 3, wherein said rebuildmodule reads from said backup storage device data stored in said faileddisk drive and backed up in said backup storage device, writes the datato said replacement disk drive; and after the completion of writing ofthe data in said failed disk drive to said replacement disk drive fromsaid backup storage device, said rebuild module refers to said modifymap, and generates data for said failed disk drive which have beenupdated after the backup, based on the data on other disk drivesconstituting said disk array, to write the generated data to saidreplacement disk drive.
 5. The disk array system according to claim 2,wherein said modify map has an update flag corresponding to a unitregion for access in each of said disk drives in said disk array, theupdate flag indicating whether the unit region has been updated or notafter the backup.
 6. The disk array system according to claim 5, whereinsaid modify map has a rebuild flag corresponding to the unit region foraccess in said each of said disk drives in said disk array, the rebuildflag indicating whether rebuilding of the unit region has been performedor not.
 7. The disk array system according to claim 6, wherein saidrebuild module refers to said modify map after the completion of writingof the data in said failed disk drive to said replacement disk drivefrom said backup storage device, and generates data on the unit regionwith the update flag indicating updating and with the rebuild flagindicating that the rebuilding has not been performed yet, based on thedata in other disk drives constituting said disk array, to write thegenerated data to said replacement disk drive.
 8. The disk array systemaccording to claim 1, wherein said rebuild module refers to a modifymap, and performs control so that writing back from said backup storagedevice to said replacement disk drive is not performed on a unit regionof the failed disk drive updated after the backup.
 9. The disk arraysystem according to claim 1, wherein an update to the failed disk duringrebuilding of the replacement disk is written to the replacement diskdrive without alteration.
 10. The disk array system according to claim1, wherein, even before the rebuilding is finished, when an accessrequest is made to latest data among the data in said replacement diskdrive under the rebuilding, access to said replacement disk drive ispermitted.
 11. The disk array system according to claim 1, furthercomprising: a disk access bus/switch for providing the RAID module andthe rebuild module with parallel access to the disk array and thereplacement disk, respectively; wherein the rebuilding of saidreplacement disk drive from said backup storage is performed throughsaid rebuild module and said disk access bus/switch, thereby therebuilding of said replacement disk drive being prohibited frominfluencing a normal access by said RAID module to said disk array. 12.The disk array system according to claim 1, wherein the rebuild moduleand the RAID module are separate from each other.
 13. A method ofrebuilding a disk array system, said method comprising: backing up datain a disk array including a plurality of disk drives onto a backupstorage device; and when a failed disk drive among said disk drivesconstituting said disk array is replaced with a replacement disk drive,rebuilding data in said replacement disk drive from the backed-up datain the backup storage device while simultaneously providing otherdevices with access to the disk drives that have not failed.
 14. Themethod according to claim 13, further comprising: recording informationcorresponding to an update location, when the data in said disk array isupdated by an access from a host after the backup onto said backupstorage device; sending only the updated data to said backup storagedevice after the backup onto said backup storage device; and storing theupdated data in said backup storage device as the backup data.
 15. Themethod according to claim 14, further comprising: reading from saidbackup storage device data stored in said failed disk drive and backedup in said backup storage device, and writing the data to saidreplacement disk drive; and referring to a modify map, after writing ofthe data in said failed disk drive to said replacement disk drive fromsaid backup storage device is finished, generating data on said faileddisk drive updated after the backup, based on the data on other diskdrives constituting said disk array and writing the generated data tosaid replacement disk drive.
 16. The method according to claim 14,wherein a modify map has an update flag corresponding to a unit regionfor access in each of said disk drives in said disk array, the updateflag indicating whether the unit region has been updated or not afterthe backup.
 17. The method according to claim 16, wherein said modifymap has a rebuild flag corresponding to the unit region for access insaid each of said disk drives in said disk array, the rebuild flagindicating whether rebuilding of the unit region has been performed ornot.
 18. The method according to claim 17, further comprising: referringto said modify map, after writing of the data in said failed disk driveto said replacement disk drive from said backup storage device isfinished; generating data on the unit region with the update flagindicating updating and with the rebuild flag indicating that therebuilding has not been performed yet, based on the data in other diskdrives constituting said disk array; and writing the generated data tosaid replacement disk drive.
 19. The method according to claim 13, therebuilding data in said replacement disk further comprising referring toa modify map and performing control so that writing back from saidbackup storage device is not performed on a unit region of the faileddisk drive updated after the backup.
 20. The method according to claim13, further comprising performing control so that a content updatedduring the rebuilding of said replacement disk drive is written to saidreplacement disk drive without using said backup storage device.
 21. Themethod according to claim 13, further comprising performing control sothat even before the rebuilding is finished, when an access request ismade to latest data among the data in said replacement disk drive underrebuilding, access to said replacement disk drive is permitted.
 22. Themethod according to claim 13, further comprising: providing a host withnormal read/write access to the disk array in parallel with the accessto the replacement disk drive for the rebuilding of data in thereplacement disk.